KABUL, Afghanistan — Three foreigners working for the American-led military coalition were among 12 people killed Saturday when their vehicle was targeted by a suicide car bomber, according to Afghan and American military accounts.

The three foreign victims were Americans working for DynCorp International, a military contractor, according to a company official in Kabul, who spoke on the condition of anonymity because he was not authorized to speak to the news media.

It was believed to be the worst loss of American life in a single episode this year in Afghanistan, since American forces have mostly withdrawn, leaving less than 10,000 soldiers.

The blast took place in the center of Kabul, the latest in a series of major bombings in recent weeks, but there was no immediate word on who was responsible.

The attack came late in the afternoon when convoys often go through downtown Kabul taking foreign and Afghan workers, as well as international military personnel, to their homes or barracks. A witness at the scene, Abdul Rahman Arif, 38, his hands and clothing covered in blood from carrying victims to ambulances, said he saw at least three bodies after the blast, and four severely wounded occupants of the coalition vehicle, as well as many children wounded in a nearby playground.

A six-story office building across the street from the blast had all of its windows broken, with many inside wounded from broken glass. Personnel at the nearby Shinozada Hospital were among those wounded by glass.

Witnesses said the bomber had been in a Toyota Corolla and appeared to have targeted a sport utility vehicle full of foreigners wearing civilian clothes but heavily armed.

Col. Brian Tribus, a spokesman for the American-led military coalition here, said three civilian contractors working for the coalition were killed; one died immediately and two died later from their wounds. He did not disclose their nationalities or jobs.

A spokesman for the Ministry of Public Health, Mohammad Ismail Kawosi, said the death toll was 12 with 67 wounded. But the Kabul police chief, Abdul Rahman Rahmani, said in an interview at the scene that the casualty toll, which included women and children, was likely to rise because of all the injuries.

Around the blast site bystanders expressed anger at the Afghan government and its international coalition partners, including the United States. “This government is nothing but a total failure,” Mr. Arif said.

Many also criticized the Afghan government’s security forces for rescuing the foreign victims in the sport utility vehicle before wounded Afghans. A woman was heard screaming at a policeman, “Have you lost your pride and respect, giving priority to foreigners while Afghans are lying dead?”

Those reactions echoed expressions of anger when three major bombings took place in a 24-hour period two weeks ago in Kabul, killing scores. Many blamed the attacks on their coalition government’s internal bickering — it has yet to appoint a permanent defense minister after 11 months in office.

Among the dead were at least two women, and a child could be heard screaming, “Mommy, Daddy, help me.”

The bombing appeared to catch the Taliban by surprise, as did some of the attacks two weeks earlier, and a spokesman, Zabihullah Mujahid, denied that the insurgents had anything to do with the attack.

That may reflect confusion within the insurgents’ ranks, in the wake of a struggle over who would succeed Mullah Mouhammad Omar, the longtime Taliban leader whose death in 2013 was disclosed only in the past month. In addition, Jalaluddin Haqqani, the leader of the hard-line Haqqani Network faction of the Taliban, recently has been reported to have died last year. That group has been responsible for many of the deadliest attacks in Kabul.

Bombings in Kabul, with its heavy security, have been relatively infrequent, typically one a month of any significance, so the recent attacks are unusual. Some analysts have speculated that the insurgents are trying to prove their leadership problems have not incapacitated them.

Fazal Muzhary contributed reporting from Kabul.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Kabul Bombing Kills 12, Including 3 Americans . Order Reprints | Today’s Paper | Subscribe